A short video:
You can edit your personal phone number on the "Co-workers & Groups" Page. An explanation with images is given below -
Go to the 'Co-workers & Groups" page on the dashboard and then click on the gear icon right next to your (or whoever's) name
A pop-up opens up. Enter the user details and number. (Enter 0 followed by mobile number)
Once you have entered these details, you will find that your number is configured. Also, to verify the newly added number, you need to place a missed call from this newly added number to 09513885656.